This application relates to the field of electronics and more particularly to monolithic microwave integrated circuit (MMIC) balun transformers.
Transformers are electric devices that transfer electric energy from one alternating-current circuit to one or more other circuits to increase (step-up) or diminish (step-down) the voltage. This transfer of energy is accomplished through electromagnetic mutual induction: a time varying current through the primary conductor produces a time varying magnetic flux through the secondary conductor. As a consequence of Faraday""s Law of Induction, this changing flux induces an electromotive force (emf) in the secondary conductor that gives rise to a current. The voltage in the secondary conductor is typically given by the ratio of the number of windings of the secondary conductor to the number of windings in the primary conductor multiplied by the voltage of the primary conductor. If this so-called turn""s ratio is greater than unity, the result is a step-up transformer that augments the primary voltage; if it is less than unity the result is a step-down transformer.
Balun transformers, devices for matching an unbalanced line (coax) to a balanced load (e.g., an antenna), have various applications. For example, modern wireless communications products such as cellular telephones, satellite receivers, and pagers require balun transformers to change from single ended to differential circuit forms in critical circuit functions such as balanced mixers. Balun transformers are needed to change impedence levels between stages while maintaining DC isolation between stages of a differential circuit. Balun transformers also find use in transmitters, providing signal isolation between local oscillators and radio frequency (RF) and intermediate frequency (IF) sections of a balanced upconverter, or coupling output pagers require balun transformers to change from single ended to differential circuit forms in critical circuit functions such as balanced mixers. Balun transformers are needed to change impedence levels between stages while maintaining DC isolation between stages of a differential circuit. Balun transformers also find use in transmitters, providing signal isolation between local oscillators and radio frequency (RF) and intermediate frequency (IF) sections of a balanced upconverter, or coupling output stages of a push-pull power amplifier. Other applications include discriminators, phase detectors, and antenna feeds.
It is desirable to perform the function of a balanced transformer with a high degree of electrical symmetry to maintain circuit isolation, and to provide broadband frequency response. It is also advantageous that these performance features be achieved with a device that is compatible with other components manufactured in monolithic microwave integrated circuit (MMIC) form, to achieve integration of balanced circuits.
MMIC balun transformers have been made in the past by first placing the primary and secondary conductors beside each other on the surface of a GaAs substrate, and then wrapping up this structure to produce a two-conductor spiral as shown below in FIG. 2. However, a problem arises with this approach because both conductors are in intimate contact with the substrate. As a result, this coplanar geometry gives rise to substantial inter-winding capacitance, which limits the operating bandwidth of the device. This effect is most dramatic in step up transformer applications, which employ a high secondary impedance level, and is most sensitive to stray capacitive loading effects. Trying to overcome this problem by increasing the spacing between the conductors to reduce the capacitance has the drawback of increasing the area occupied by the transformer, and also reducing the magnetic coupling coefficient between the primary and secondary conductors. Moreover, these known approaches do not give rise to the symmetrical circuits important in balanced devices such as upconverters, downconverters, and frequency multipliers. Taking a different tack of using active circuit techniques to develop balanced signals gives rise to other problems including consuming precious battery current, injecting noise, and not exhibiting true broadband symmetry.
As mentioned above, transformers rely on the principle of mutual inductance whereby a changing current in a primary conductor gives rise to an emf in a secondary conductor. Inductors, on the other hand, which only have a single conductor, rely on self inductance to operate, and are important electronic devices in their own right. The most common type of inductor consists of a single coil of a conductor, the windings of which are tightly packed together along the surface of a cylinder. The tightly wound turns of the conductor help increase the inductance: each of the windings contributes an emf and also contributes to the magnetic flux through the coil. Hence, increasing the number of windings by a factor of N increases the self-inductance by a factor of N2. Thus, any innovation that can pack more windings into less space would give rise to a more desirable inductor. It is desirable to reduce the inter winding capacitance between the turns of an inductor to increase its self resonant frequency, and thusly increase the bandwidth over which the component behaves substantially as an inductor.
The invention addresses the aforementioned problems by providing balun transformers that place the secondary windings of the transformer above the primary windings on supporting posts. In one particular embodiment, the balun transformers described herein have two tiers of octagonal windings, one mounted on top of the other. The technology used to achieve this configuration is the same MMIC technology commonly used for crossovers. This MMIC technology is well established and can be found, for example, in many GaAs foundries used to make contacts to the top plate of thin film capacitors. Thus, no costly processing steps employing additional insulating layers (and increasing the primary to secondary capacitance) are required to realize the benefits of a symmetrical balun transformer.
With minor modifications, the present invention can also result in an inductor. In particular, by connecting the primary and secondary conductors in series, the result is a new inductor that is able to pack an increased number of windings into a defined, small area leading to desirable electronic characteristics, such as large inductances. Often, increasing the number of windings by a factor of N, increases inductive effects by a factor of N2, as is the case for the common inductor, composed of a close-packed helical coil, which can be formed by tightly winding a conducting wire around a cylinder""s surface. For such an inductor, it is well known that the inductance is given, in MKS units, by xcexcn2 1A, where xcexc is the permeability constant, n is the number of windings per unit length, and 1 and A are the length and cross-sectional area of the coil, respectively. Thus, being able to pack more windings in a given volume can be expected to yield significant increases in the inductance.
More particularly, the systems described herein can include balun transformers having primary and secondary conductors, with a dielectric in between, such that each of the conductors lies in different, but parallel planes. The conductors have windings to promote the exchange of electromagnetic energy via mutual inductance. The system described herein can also include a balun transformer having an approximately planar secondary conductor, such that the secondary conductor has a large inductance arising from winding the secondary conductor in a relatively small volume of space that provides more effective flux linkages.
In various embodiments, the secondary conductor can be supported by posts standing on pedestals, the dielectric can be air, the windings of the primary conductor can be wider than those of the secondary conductor, or curve beneath the secondary conductor to avoid the pedestals, and the primary and secondary conductors can be separated by a gap of the order of a micron (10xe2x88x926 meters). The secondary conductor can include windings along concentric polygons, such as octagons, or circles, with crossover points between adjacent tracks.
The systems described herein can also include inductors having upper, secondary and lower, primary conductors electrically connected in series, and separated by at least one dielectric, such that the lower, primary and upper, secondary conductors lie substantially on parallel planes and have more than one winding of a conductor.
In various embodiments, the inductor can have an upper, secondary conductor that lies on supporting posts on pedestals. The inductor can have a lower, primary conductor that both curves beneath the upper, secondary conductor so as to avoid the pedestals, and windings that are wider than windings of the upper, secondary conductor. The dielectric between the lower, primary and upper, secondary conductors can be air, silicon nitride, or polyimide, for example.
Described herein is also a method for making balun transformers including laying a planar primary conductor below a planar secondary conductor, and leaving a gap between the first and second conductors that can be filled with a dielectric.
In variants of the method, the secondary conductor can lie on supporting posts on pedestals, the primary conductor can have windings that are wider than those of the secondary conductor, and curve beneath the secondary conductor so as to avoid the supporting posts, and the dielectric can be air.
Described herein is also a method for making inductors which involves laying a substantially planar lower, primary conductor next to a substantially planar upper, secondary conductor, connecting the lower, primary and upper, secondary conductors in series, leaving a gap between the lower, primary and upper, secondary conductors, and providing at least one dielectric to reside in the gap.
In variants of the method, the upper, secondary conductor lies on supporting posts on pedestals, and the lower, primary conductor, which can have windings that are wider than those of the upper, secondary conductor, curves beneath the upper, secondary conductor in order to avoid the pedestals. The dielectric separating the two conductors can be air.
Additionally, a method of compactly winding a secondary conductor for a balun transformer is described herein. The method involves coiling the conductor along concentric polygons or circles substantially in a plane, and providing crossovers to adjacent tracks.
In variants of the method, the polygons can be octagons, the crossover points can occur at every half revolution of the polygons, and the secondary conductor can lie on posts standing on pedestals. In addition, the method can include disposing a primary conductor beneath the secondary conductor wherein the primary conductor substantially shadows the windings of the secondary conductor from the ground plane on the underside of the MMIC chip and curves to avoid the pedestals.
Benefits of the described invention include reduction in chip area with a concomitant reduction in price; increase in performance by increasing bandwidth, reduction in gain/power variations with frequency, improvements in balance and spurious signal output; new products, such as broadband, general purpose frequency converters; and simplification and performance enhancements of system level products with improved balance in mixers leading to fewer spurious signals, and a reduction in requirements for high Q external filters and a simplification in frequency plans for broadband radios.